The present invention relates to a printing system and, more particularly, to a printing system in which multiple-page documents may be duplex printed.
A number of different prior art printing systems have had a duplex printing capability. For example, Wick U.S. Pat. No. 4,066,252 and Satomi et al U.S. Pat. No. 4,218,130 disclose xerographic copiers including an arrangement for transporting a copy sheet past a print station, inverting the sheet, and then transporting it past the print station a second time. In the Wick '252 system, after a sheet is printed on a first side by contact with a photosensitive belt and fixation of the transferred image, it is transported to an output accumulation tray. The sheet is then withdrawn from the output accumulation tray and re-introduced into the paper path, while at the same time being turned side for side. The sheet is reapplied to the photo-sensitive belt with its second side in contact with the belt, and printing on this side is accomplished.
It is significant that only one sheet is present at any time in the copy sheet paper path. After printing on the first side of a sheet it is transported to the output accumulation tray, effectively inverted, returned to the print station, and then transported again to the output accumulation tray. Only at this point is the next successive sheet of copy paper supplied to the paper path. As a consequence, the duplex printing speed of the Wick '252 system is limited by the fact that a substantial amount of time is required between successive printing operations while a sheet is inverted, or removed from the paper path and a new sheet supplied to the paper path. The actual printing operation occupies a relatively small portion of the time required for producing each duplex printed sheet.
The Satomi et al '130 patent discloses a xerographic copier similar to that of Wick '252. In the Satomi et al device, however, after one side of a sheet is printed, the sheet is supplied to a sheet inverting tray which is separate from the output accumulation tray output. Supplying the sheet to this tray and then removing it from the tray results in the sheet being inverted as it is reintroduced into the paper path. As in the device disclosed in the Wick '252 patent, the first embodiment of Satomi et al '130 permits only one copy sheet to circulate through the paper path at any time and, therefore, this embodiment is limited in its operating speed. In an alternative embodiment, the Satomi et al '130 patent discloses a duplex printer which has two intermediate duplex trays. Sheets of copy paper are stacked in these trays after printing the first sides thereof and prior to returning the sheets to the print station for printing on the second sides of the sheets. This arrangement is utilized when it is desired to print duplex copies with the copies having front and back images not corresponding to the front and back images on the pages of the original document. Such intermediate storage is required since the xerographic copying device disclosed does not provide an arrangment for image storage prior to printing. It will be appreciated, however, that stacking the partially printed sheets may be undesirable, especially where there is a possibility of inadvertent print image transfer between the stacked sheets, or smearing of the print images during stacking and refeeding.
DiFrancesco et al U.S. Pat. No. 4,174,905 discloses a printer arrangement for making duplex copies. Each sheet of copy paper is supplied in sequence to a first printing station, to an inverter station, and finally to a second printing station. Opposite sides of each sheet are printed at the first and second printing stations. This device requires several print stations to accomplish duplex printing. While it is capable of higher speed operation than the above discussed Wick and Satomi et al devices, nevertheless it requires a substantial amount of space.
Accordingly, it is seen that there is a need for an improved printing system in which high speed duplex printing of a plurality of copy sheets may be accomplished at a single print station.